The term "hardcopy device" includes a variety of scanners, printers and plotters, including those using inkjet and electrophotographic technologies to read an image from, or to apply an image to, a hardcopy medium, such as paper, transparencies, fabrics, foils and the like. Inkjet printing mechanisms print images using a colorant, referred to generally herein as "ink." These inkjet printing mechanisms use inkjet cartridges, often called "pens," to shoot drops of ink onto a page or sheet of print media. Some inkjet print mechanisms carry an ink cartridge with a full supply of ink back and forth across the sheet. Other inkjet print mechanisms, known as "off-axis" systems, propel only a small ink supply with the printhead carriage across the printzone, and store the main ink supply in a stationary reservoir, which is located "off-axis" from the path of printhead travel. Typically, a flexible conduit or tubing is used to convey the ink from the off-axis main reservoir to the printhead cartridge. In multi-color cartridges, several printheads and reservoirs are combined into a single unit, with each reservoir/printhead combination for a given color also being referred to herein as a "pen." As the inkjet industry investigates new printhead designs, one trend is toward using a "snapper" reservoir system where permanent or semi-permanent printheads are used and a reservoir carrying a fresh ink supply is snapped into place on the printhead.
Each pen has a printhead formed with very small nozzles through which the ink drops are fired. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor.
To print an image, the printhead is propelled through a printzone back and forth across the page, ejecting drops of ink in a desired pattern as it moves. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). The nozzles are typically arranged in linear arrays usually located side-by-side on the printhead, parallel to one another, and perpendicular to the scanning direction of the printhead, with the length of the nozzle arrays defining a print swath or band. That is, if all the nozzles of one array were continually fired as the printhead made one complete traverse through the printzone, a band or swath of ink would appear on the sheet. The width of this band is known as the "swath height" of the pen, the maximum pattern of ink which can be laid down in a single pass. The print media, such as a sheet of paper, is moved through the printzone typically one swath width at a time, although some print schemes move the media incrementally by, for instance, halves or quarters of a swath width for each printhead pass to obtain a shingled drop placement which enhances the appearance of the final image.
Whether the printing mechanism uses either a snapper cartridge system, an off-axis system, a replaceable cartridge system or some other inkjet system, drop placement on the media must be coordinated with the incremental advance of the media through the printzone for sharp, vivid images and text, which are free of print defects, such as color banding, improper spacing, and printed line overlapping. Many types of inkjet printing mechanisms use a series of conventional paper drive rollers or tires to frictionally engage the print media and incrementally advance the media through the printzone, moving either a full or fractional swath width. To provide feedback to the printer controller regarding the location of the media with respect to the printhead, more recent printers, such as the DeskJet.RTM. 720C and 722C models of inkjet printers, manufactured by the present assignee, the Hewlett-Packard Company of Palo Alto, Calif., have incorporated an optical encoder wheel on the axle of the media advance tires. This system required two optical sensors to read the encoder wheel and correct for any eccentricity of the code wheel, as described in U.S. Pat. No. 5,774,074, which is assigned to the present assignee, the Hewlett-Packard Company. It would be desirable to implement a new media advancing and positioning system that increases printing speed and accuracy to provide consumers with a faster printing unit which prints high quality images.
Other hardcopy devices include scanners which have a scanhead with image receptors that "read" an image previously printed on media, and convert this image into an electronic file which may then be computer edited, or sent to a selected destination via either electronic mail (e-mail) or facsimile transmitted over telephone lines, for instance. The image receptors in a scanhead may be a series of discrete elements arranged in a linear array, as described above for an inkjet printhead. These hardcopy scanning mechanisms may use the same media advance system as described above for an inkjet printing mechanism, and indeed, in many multi-function devices the same media advance system is used for both printing and scanning.
As a more general concept, both inkjet printheads and scanheads may be considered as "image transceiver heads," with printheads transceiving an image by printing that image on media, while scanhcads transceive an image by "reading" an image that already exists on media. This generic image transceiver head may have one or more arrays of discrete interaction elements arranged, for instance, in a linear array, to selectively interact with media in an interaction zone of the hardcopy device. For a printing mechanism, the interaction elements are ink-ejecting nozzles and the interaction zone is a printzone. For a scanning mechanism, the interaction elements are image receptors and the interaction zone is a readzone, although in some multi-function devices, the printzone and readzone may both physically occupy the same location adjacent the media advance path.